Thermal sublimation and deposition of materials on a substrate in a vacuum is known and in increased demand especially for deposition of materials having high melting temperatures on continuous webs. Several prior art techniques are commonly used for thermal evaporation of coating materials wherein a coating material is heated to a melting temperature within a vacuum chamber creating a vapor and the vapor condenses on a cooler substrate forming a coating on the substrate. Prior art thermal evaporation systems include aluminum wire feed systems, induction heaters and heating systems having electron beam sources.
Aluminum wire feed systems normally include a spool of metal wire being continuously fed onto an electrically resistive boat which is heated. The wire coating material contacts the boat and evaporates from a solid to a vapor. The vapor then condenses onto a substrate. Wire feed thermal evaporation systems are efficient and operate at relatively high line speeds with high deposition rates and long run times, however, are generally limited to one type of a source material.
Induction heater systems normally include an open crucible containing a coating material wherein the coating material is heated indirectly by heating the crucible. The material is slowly heated usually from a solid to a liquid and finally to a temperature at which point the coating material is vaporized. The vaporized coating material is then dispersed and collected onto a substrate forming a coating thereon. One major drawback of this type of system is that the entire quantity of the coating material in the crucible must be heated to a boiling temperature before the coating process can begin. Also, an evaporation process is often limited by the capacity of the crucible. Accordingly, major drawbacks of many induction heater systems include long heat-up and cool-down times and limited material supplies causing reduced deposition rates, line speeds and overall throughput of a coating process.
Additionally, induction heating systems wherein a coating material is heated from the sides or bottom of a crucible can have a tendency to blow out vapor from the interior of the crucible causing undesirable splashing or splattering at the surface of the coating material in the crucible.
Heating systems having electron beam sources normally provide direct heat to a coating material contained in a crucible by focusing a high current electron beam on the surface of the coating material and scanning the beam over the surface thereof in order to uniformly heat the coating material in the crucible. The comparatively high cost of this type of electron beam system drastically limits its usefulness in the economical production of coated materials.
Additionally, many prior art thermal heating elements are made from expensive metals such as molybdenum, tungsten or tantalum which can be easily corroded or damaged by contacting coating materials and/or reaction gases like nitrogen and oxygen which may be added to a coating process for forming an oxide or nitride film on a substrate or for assisting the reaction process. If the material of a heating element reacts with evaporated coating material or a reaction gas, a build-up of corrosion or other materials can close or partially block openings in the heating element which can prevent a coating material from passing through the heating element inhibiting the coating process.
Based on the foregoing, it is the general object of the present invention to provide a multi-layered radiant thermal evaporator that improves upon, or overcomes the problems and drawbacks associated with the prior art.